* Update Android NDK to version r19c
* Update Android API versions to:
  * API 24 on arm and aarch64 targets
  * API 28 on x86 and x86_64 targets
* Unified headers were removed in NDK 16
* Refactor the NDK and SDK installation scripts
* OpenJDK version 8 must be kept: it appears that
  the Android tools do not work with more modern OpenJDK
  versions.

This helps caching between runs and can help speed up turnaround time for
various operations. The old android container didn't work out for some reason
due to permissions so the definition has now been vendored locally to just
rebuild it each time.

Previously we were somewhat half-Docker, half-Travis, half apt-get, etc. This
commit alters the CI infrastructure to use Docker images for each target. This
should make it much easier to update the images and tweak various bits and
pieces of installed software. Additionally, it's also much clearer now what's
needed for each suite of tests!

Some images were updated a bit (e.g. musl is now 1.1.14), but other images
encountered failures when updating so they're not getting updated just yet.